1. Field of the Invention
The present invention relates to an electron beam irradiating apparatus for processing, for example, waste gas discharged from a thermal power plant. The present invention especially relates to a window foil for ejecting electrons from a vacuum vessel into a gas environment, and particularly to a crosspiece fixed to the vacuum vessel for supporting the window foil against atmospheric pressure.
2. Description of the Related Art
Some of today global problems, such as global warming and acid rain, are thought to be caused by air pollution and specifically by SOx, NOx, and other toxic components in waste combustion gases that are discharged from thermal power plants and the like. One method for removing such toxic components, as SOx and NOx, has been to conduct desulfurization and denitration by irradiating waste combustion gases with an electron beam.
FIG. 1 shows an example of an electron beam irradiating apparatus for use in such applications. An apparatus for processing waste combustion gas includes a power source 10 for generating a high DC voltage, an electron beam irradiating apparatus 11 for irradiating an electron beam onto waste combustion gas, a window foil 15 serving as an irradiating outlet for the electron beam of the electron beam irradiating apparatus 11, and a channel 19 disposed along the window foil 15 through which the waste combustion gas flows. The window foil 15 is composed of a thin plate formed of titanium or the like.
The electron beam ejected externally via the window foil 15 irradiates such molecules in the waste combustion gas as oxygen (O2) and water vapor (H2O) to form radicals that are extremely strong oxidizers, including OH, O, and HO2. These radicals oxidize the toxic components SOx and NOx to generate intermediate products of sulfuric acid and nitric acid. The intermediate products react with ammonium gas (NH3) that has already been introduced to form ammonium sulfate and ammonium nitrate, which can be recovered for use as a fertilizer. Accordingly, this type of waste gas processing system can remove such toxic components as SOx and NOx from waste combustion gas and can recover useful by-products of ammonium sulfate and ammonium nitrate for use as a fertilizer.
In this example, the electron beam irradiating apparatus 11 includes as main components a thermoelectron generator 12 such as a thermionic filament; an accelerating tube 13 for accelerating electrons emitted from the thermoelectron generator 12; a focusing electromagnet 16 for controlling a diameter of an electron beam by applying a magnetic field to a highly energized electron beam formed by the accelerating tube 13; and a scanning electromagnet 17 for deflecting the electron beam by applying a magnetic field to the electron beam after it has been focused to a specific diameter. These components are accommodated in vacuum vessels 18a and 18b, which maintain a high vacuum atmosphere of approximately 10xe2x88x926 Pa. By applying a magnetic field using the scanning electromagnet 17, the highly energized electron beam is deflected in a scanning motion through the window foil 15 and ejected onto the waste combustion gas within a prescribed range of the channel 19.
As described above, this type of electron beam irradiating apparatus must eject an electron beam into the atmosphere after electrons have been accelerated in a vacuum environment. The window foil used in this electron beam irradiating apparatus is generally a film formed of pure titanium or a titanium alloy having a thickness of several tens of micrometers (for example, 40 xcexcm) in order to attain a high electron transmission efficiency for ejecting the electron beam. The window foil 15 is mounted on an end of the vacuum vessel 18a via a mounting flange (not shown). The size of the window foil 15 is as large as 3 mxc3x970.6 m. Here, an atmospheric pressure of approximately 1000 hPa is applied to a surface of the window foil, against an internal pressure of approximately 10xe2x88x926 Pa in the vacuum vessel. Accordingly, a large force is applied to the window foil via a relationship of area and pressure differential. Therefore, a crosspiece is affixed to a portion of the window foil surface, thereby dividing the window foil into a plurality of sections.
FIG. 2 shows an example of a construction for dividing the window foil 15 with a crosspiece, and a scanning path for an electron beam. As described above, the window foil 15 is relatively large, i.e. 3 m by 0.6 m. A central crosspiece 21 is disposed lengthwisely across a center of the window foil 15 and adhered thereon for supporting the same. Accordingly, the crosspiece 21 supports a central portion of the window foil 15 and divides the surface of the window foil 15 into two sections. This configuration prevents the window foil 15 from deforming, even while incurring a large pressure from an atmospheric side toward an evacuated side of the window foil.
Since the electron beam is emitted over a large range through the window foil 15, which has a relatively large area, it is possible to avoid heat damage to the window foil 15. Therefore, the electron beam scans the window foil 15 in lengthwise direction thereof along a path P in the direction shown by the arrow in FIG. 2 in order to avoid heat damage to the window foil 15. At this time, the electron 20 beam scans along a path that does not traverse the crosspiece in the lengthwise direction. The electron beam is accelerated by a voltage of approximately 800 kV. Even with a large electric current of approximately 500 mA, energy loss occurs when the electron beam passes through the window foil, thereby generating a large amount of heat therein. For this reason, a cooling pipe is provided in the crosspiece 21 and cooling water is flowed through the pipe to prevent damage to the window foil 15 due to excess heat.
However, in order to scan the window foil 15 along the path P shown in FIG. 2, the electron beam inevitably must cross end portions 21a and 21b. When the electron beam E crosses these portions and irradiates the end portions 21a and 21b, a large amount of heat is generated on these portions of the crosspiece 21. Over extended use, this excess heat can cause thermal fatigue in portions of the cooling water pipe provided in the crosspiece 21, leading to such accidents as water leaking into the evacuated section. If the vacuum vessel becomes contaminated, it is necessary to disassemble the electron beam irradiating apparatus to repair the damage and then reassemble the apparatus. This process requires much time and labor. Further, once the damage has been repaired, more time is required to evacuate the vacuum vessel again, thereby requiring that operations be suspended for a considerably long time, in addition to difficult repair work.
In view of the foregoing, it is an object of the present invention to provide an electron beam irradiating apparatus for scanning an electron beam across an entire surface of a window foil, wherein the window foil is reinforced by a crosspiece capable of preventing excess heat from being generated due to electron beam irradiation, thereby enabling the apparatus to perform stable operations for an extended time. It is another object of the present invention to provide such an electron beam irradiating apparatus that is easy to maintain.
These objects and others will be attained by an electron beam irradiating apparatus comprising an electron beam source for emitting electrons; an accelerating unit for accelerating electrons emitted from the electron beam source to generate a highly energized electron beam; a deflecting unit for deflecting the highly energized electron beam generated by the accelerating unit in a scanning direction; a vacuum vessel accommodating the electron beam source, the accelerating unit, and the deflecting unit in a vacuum environment; a window foil for ejecting the electron beam from the vacuum environment into a gas environment; a crosspiece for adhering to and supporting the window foil; and a cooling block for shielding the crosspiece from the electron beam in areas that the electron beam intersects the crosspiece.
With this construction, the cooling block can prevent excess heat from being generated in portions of the crosspiece over which the electron beam passes by receiving irradiation of the electron beam at these portions. This construction allows stable operation of the electron beam irradiating apparatus.
According to another aspect of the present invention, the cooling block can be detachably mounted. With this construction, the cooling block can be easily replaced after incurring fatigue by irradiation of an electron beam over a long period of time. Therefore, the present invention can prevent such serious accidents as water leakage and the like through regular replacement of the cooling block.
According to another aspect of the present invention, the cooling block includes a cooling pipe formed of a metal material and has a fluid therein. With this construction, the cooling block can be easily manufactured, and a portion of the cooling block irradiated by an electron beam can be easily cooled.
According to another aspect of the present invention, a cross-section of the cooling block is gently curved on a side facing the electron beam source. With this construction, an electron beam evenly irradiates a surface of the cooling block, thereby preventing excess heat from energy of the electron beam from being concentrated on a portion of the block.